THE DIRECT MEASUREMENT OF THE EXTRACELLULAR PHASE OF TISSUES 12

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Abstract
Measurements of the extracellular fluid in the human subject by inulin and thiosulfate give volume distrs. so small that they are difficult to reconcile, biochemically or morphol., with balance data and tissue analysis in experimental animals and man. To clarify this problem, the distr. of a single injn. of inulin and thiosulfate was measured directly in the tissues of 8 nephrectomized dogs, at varying time intervals, and compared with simultaneous electrolyte, water, and collagen analyses of these tissues. In all the tissues studied, the chloride space exceeded the thiosulfate space, and this exceeded the inulin space. Dense connective tissue, such as tendon, showed an increase in size of the thiosulfate and inulin spaces from 3 to 6 hrs. At 6 hrs. thiosulfate space represented 75% of the extracellular phase; inulin, 25%. Thiosulfate space was 60% of chloride space in muscle, 75% in skin, and showed no increase from 3 to 6 hrs. Inulin spaces in muscle and skin increased slightly from 3 to 6 hrs., reaching 50% of the chloride space. Since neither thiosulfate nor inulin spaces reach the size of the plasma ultrafiltrate phase of tendon or skin in 6 hrs., they are not accurate measures of the extracellular fluid within this time interval. However, from a morphol. point of view, inulin or thiosulfate space in tissues, plus that fraction of the connective tissue phase which remains unpenetrated at any given time, defines a space which is only slightly smaller than the space defined by chloride, suggesting that when chloride space is corrected for the higher concn. of chloride in connective tissue, it is both biochemically and morph. the best measure of the extracellular space.
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